Why did the committee of public information publish propaganda posters depicting germans as ferocious beasts?

You are probably talking about the US Committee on Public Information, which operated for 26 months from 1917 to 1919 during World War I. It is an independent agency of the U. S government, in the level of CIA, FCC, FTC, and the Postal Service. The agency was tasked to use every medium available to build up public enthusiasm to the war effort. It did so by engaging in a prolonged propaganda campaign to influence American public opinion and support war effort, often depicting Germans as beasts. The committee was disestablished soon after the Armistice was signed on November 11, 1918.